1. Field of the Invention
The present invention relates to improved elastomeric stator assemblies.
2. Description of the Related Art
Progressive cavity pumps having elastomeric stator assemblies have been used in drilling operations for some time. In downhole drilling operations, progressive cavity motors pump fluids by spinning a rotor within a stator assembly. In particular, power sections for mud motors have seen extensive use of progressive cavity pumps in the inverse application where drilling fluid is used to turn a rotor within a stator. Often, progressive cavity motors are configured with helical metal lobed rotors that turn within elastomeric stators consisting of rubber with high carbon black filler content. The high carbon black rubber provides a suitable yet cost efficient material having some compressive modulus and abrasion resistance properties. As the metal lobes of the rotors press against the elastomeric stator inner walls, a sealing line is formed and fluid is thus pumped through the cavities as they are formed between the metal lobes of the rotors and the elastomeric stator inner walls.
A challenge to producing a high power, high torque, and high speed power section stator, is that manufacturing equipment and cost effective tooling materials require a low viscosity uncured elastomer compound that is capable of flowing through a tight mould cavity over a long distance while maintaining its uncured state. If a compound is too viscous it cannot flow the appropriate distance to fill the mould. If a compound begins the vulcanization reaction before the mould is filled, the compound will increase in viscosity exponentially, possibly resulting with a mould that is not filled, or a mould that will fill with cross-links that form in separate matrices. Separately formed matrices create undetectable grain boundaries in the elastomer product which will often fail prematurely due to significant losses in tear resistance, losses in modulus, and or friction points internally that facilitate rapid physical deterioration of the surrounding elastomer matrix. Traditionally, designers of power section elastomers have sought to address these issues using reinforcing and semi-reinforcing carbon blacks, low viscosity low molecular weight base NBR and HNBR polymers, and process aids in the recipe. Although such combinations are favorable for manufacturability, the resulting recipe negatively impacts the final cured state properties of the elastomer, often making the formulation softer and less dynamically stable. For example, plasticizer oil may be used to decrease viscosity during manufacturing but, in the finished product, it has a tendency to leach out of the elastomer at high temperatures when exposed to diverse drilling fluids, which can cause shrinkage in the product or de-bonding of the rubber to metal bonding agents, and also facilitate the absorption of chemicals from drilling fluid. Plasticizers are used to reduce the viscosity of an uncured rubber compound by lubricating between the polymer chains and aiding in the dispersion of carbon blacks. Once in cured state, plasticizers continue to lubricate the polymer chains creating an effect of lowered modulus. Additionally, plasticizers, being significantly lower molecular weight than polymers, can migrate out of a compound. Controlling the migration of plasticizers is a function of choosing a plasticizer with the right molecular mass/branching and carbon-to-oxygen ratio for a particular compound. The more branching a plasticizer has, the more resistant the plasticizer is to fluid extraction in oils. The potential to react an ester-based plasticizer into the polymer matrix will substantially increase the resistance to extraction. If plasticizers can be peroxide cross-linked to a polymer, then the same plasticizer can be cross-linked to a functionalized graphene particle, permanently locking the plasticizer into the polymeric matrix. By reacting a plasticizer into the polymers and graphene particles, higher than traditional loadings of plasticizer can be compounded into a recipe without the drawback of having plasticizer leaching out of the compound to the rubber to metal bond of the stator or out of the inner diameter surface. Higher loadings of plasticizer offset the high viscosity effect that nano-particles induce in a compound. Higher loadings of plasticizer also reduce the mixing times of uncured compound by aiding the peptization and dispersion of filler materials like carbon blacks and nanoparticles. By reducing the mixing times and shear rates, less polymer is broken in the mixing process, which will improve physical and dynamic properties such as: modulus, tensile strength, tear resistance, tan delta, shear modulus, compressive modulus and hardness.
Elastomeric compounds have seen the incorporation of phenolic resins as referred to in U.S. Pat. App. Pub. No. US 2008/0050259, which reduces the uncured viscosity of the compound and increases the hardness of the cured state product at the cost of reduced tear resistance.
Elastomeric compounds have also seen some use of nanoparticles; however, due to the extraordinary surface area to particle volume (i.e. aspect ratio), these compounds can greatly increase the viscosity of the elastomer with only small amounts of additive nanoparticles. This means their potential in power sections stator compounds requires such low loadings (to maintain manufacturability) that the cured state physical properties are not attainable at an affordable, reproducible level of satisfaction.
Further, though the helical metal rotors of progressive cavity motors are heat tolerant, abrasion resistant, and have generally long useful lives, the stators of progressive cavity motors are far less reliable and often fail, need servicing, or replacement before their rotor counterparts. The carbon black reinforced liner of stators tends to wear down when exposed to abrasive materials, can develop leaks between cavities. When exposed to harsh temperatures a rubber compound will soften and can result in seal lines being less capable of handling high differential pressure which can result in a loss in torque. High temperatures can also cause the rubber in a liner to thermally expand, which can lead to overheating. Long term exposure to such conditions can cause the rubber to become brittle and lead to low tear resistance. Failures can occur in the form of a section worn down by abrasion leaking and not providing proper sealing pressure against the metal rotor lobes; a physical tear of the inner lining can also occur and cause an immediate shutdown of the entire system. For example, when the stator fails, the rotor can pump torn rubber pieces through cavities and damage other components of the downhole assembly or stop rotating all together. Exposure to certain chemicals or downhole fluids can additionally cause degradation of the stator inner walls. Harsh drilling fluids can be absorbed into the rubber liner, causing swelling which leads to the rubber liner overheating in operation. Fluids can also extract chemicals from the rubber, thereby degrading it.
It would thus be desirable to have a more robust elastomeric stator assembly with increased heat tolerance, abrasion resistance, tear resistance, and other beneficial properties. Further, it would be desirable to provide increased meantime between failures, increased reliability, and an expectation of extended runtime for operations running elastomeric stator assemblies downhole. This would allow greater drilling time and decreased time spent installing, retrieving, and servicing elastomeric stator assemblies and other components of the associated downhole assemblies that can fail as the result of a stator failure. It would further be desirable to increase the predicted time interval between required servicing of elastomeric stator assemblies.